Glass for chemical tempering and glass plate for display device

ABSTRACT

To provide glass to be used for chemically tempered glass which is hardly broken even when flawed. 
     Glass for chemical tempering, which comprises, as represented by mole percentage based on the following oxides, from 65 to 85% of SiO 2 , from 3 to 15% of Al 2 O 3 , from 5 to 15% of Na 2 O, from 0 and less than 2% of K 2 O, from 0 to 15% of MgO and from 0 to 1% of ZrO 2 , and has a total content Si0 2 +Al 2 O 3  of SiO 2  and Al 2 O 3  of at most 88%.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/619,290 filed on Sep. 14, 2012, the text of which is incorporatedherein by reference, and which is a continuation of PCT Application No.PCT/JP2011/061454, filed on May 18, 2011, which claims the benefits ofpriorities from Japanese Patent Application No.2010-115365 filed on May19, 2010, Japanese Patent Application No.2010-278106 filed on Dec. 14,2010, and Japanese Patent Application No.2010-288255 filed on Dec. 24,2010. The contents of those applications are incorporated herein byreference in their entireties.

TECHNICAL FIELD

The present invention relates to a display device such as a mobiledevice such as a cell phone or a personal digital assistance (PDA), atouch panel or a large-sized flat screen television such as alarge-sized liquid crystal television, a glass plate for a displaydevice suitable for e.g. a cover glass for a display device, and glassfor chemical tempering suitable for such a glass plate.

BACKGROUND ART

In recent years, for a display device such as a mobile device such as acell phone or a PDA, a touch panel or a liquid crystal television, acover glass (protective glass) has been used in many cases in order toprotect the display and to improve the appearance. Further, to a coverglass for a flat screen television such as a liquid crystal television,surface treatment, for example, formation of a film having a functionsuch as antireflection, impact failure prevention, electromagnetic waveshielding, near infrared ray shielding or color tone correction may beapplied in some cases.

For such a display device, weight reduction and thickness reduction arerequired for differentiation by a flat design or for reduction of theload for transportation. Therefore, a cover glass to be used forprotecting a display is also required to be thin. However, if thethickness of the cover glass is made to be thin, the strength islowered, and there has been a problem such that the cover glass itselfis broken by e.g. a shock due to falling or flying of an object in thecase of an installed type or by dropping during the use in the case of aportable device, and the cover glass cannot accomplish the essentialrole to protect a display device.

In order to solve the above problem, it is conceivable to improve thestrength of the cover glass, and as such a method, a method to form acompressive stress layer on a glass surface is commonly known.

The method to form a compressive stress layer on a glass surface, maytypically be an air quenching tempering method (physical temperingmethod) wherein a surface of a glass plate heated to near the softeningpoint is quenched by air cooling or the like, or a chemical temperingmethod wherein alkali metal ions having a small ion radius (typically Liions or Na ions) at a glass plate surface are exchanged with alkali ionshaving a larger ion radius (typically K ions) by ion exchange at atemperature lower than the glass transition point.

As mentioned above, the thickness of the cover glass is required to bethin. However, if the air quenching tempering method is applied to athin glass plate having a thickness of less than 2 mm, as required for acover glass, the temperature difference between the surface and theinside tends not to arise, and it is thereby difficult to form acompressive stress layer, and the desired property of high strengthcannot be obtained. Therefore, a cover glass tempered by the latterchemical tempering method is usually used.

As such a cover glass, one having soda lime glass chemically tempered iswidely used (e.g. Patent Document 1).

Soda lime glass is inexpensive and has a feature that the surfacecompressive stress S of a compressive stress layer formed at the surfaceof the glass by the chemical tempering can be made to be at least 550MPa, but there has been a problem that it is difficult to make thethickness t of the compressive stress layer (hereinafter sometimesreferred to as the compressive stress layer depth) to be at least 30 μm.Glass in the after-mentioned Example 27 is soda lime glass.

Therefore, one having SiO₂—Al₂O₃—Na₂O type glass different from sodalime glass, chemically tempered, has been proposed for such a coverglass (e.g. Patent Documents 2 and 3).

Such SiO₂—Al₂O₃—Na₂O type glass (hereinafter referred to as conventionalglass) has a feature that it is possible not only to make the above S tobe at least 550 MPa but also to make the above t to be at least 30 μm.Glasses in the after-mentioned Examples 28 and 29 are conventionalglasses.

PRIOR ART DOCUMENTS Patent Document

Patent Document 1: JP-A-2007-11210

Patent Document 2: U.S. Patent Application Publication No. 2009/0298669

Patent Document 3: U.S. Patent Application publication No. 2008/0286548

DISCLOSURE OF INVENTION Technical Problem

It is highly possible that a mobile device is dropped from the user'shand, pocket or bag and its cover glass gets flaws (indentations), orthe dropped mobile device may be stepped on or the user may sit on themobile device put in the pocket, and accordingly a heavy load may beapplied to the cover glass in many cases.

A flat screen television such as a liquid crystal television or a plasmatelevision, particularly a large-sized flat screen television having asize of at least 20 inches, is likely to have flaws since its coverglass has a large area, and as the screen is large, the probability ofbreakage from the flaws as the breakage origin is high. Further, when aflat screen television is used on the wall, it may fall down, and insuch a case, a significant load is applied to the cover glass.

A touch panel is likely to have flaws such as scratches at the time ofits use.

As such large or small display devices are used more widely, the numberof breakage events of the cover glass itself is increased as comparedwith a case where the number of use was small.

It is said that a chemically tempered cover glass made of conventionalglass which is used at present is broken when a force of 5 kgf=49 N isapplied by means of a Vickers indenter of a Vickers hardness meter.

It is an object of the present invention to provide a glass plate for adisplay device which is less likely to be broken even when flawed ascompared with conventional product.

Solution Problem

The present invention provides glass for chemical tempering, whichcomprises, as represented by mole percentage based on the followingoxides, from 65 to 85% of SiO₂, from 3 to 15% of Al₂O₃, from 5 to 15% ofNa₂O, from 0 and less than 2% of K₂O, from 0 to 15% of MgO and from 0 to1% of ZrO₂, and has a total content SiO₂+Al₂O₃ of SiO₂ and Al₂O₃ of atmost 88% (hereinafter this glass for chemical tempering will sometimesbe referred to as glass of the present invention). In thisspecification, “from 65 to 85%” for examples means “at least 65% and atmost 85%”, and “from 0 and less than 2%” means “at least 0% and lessthan 2%”.

The present invention further provides glass for chemical tempering,which comprises, as represented by mole percentage based on thefollowing oxides, from 68 to 80% of SiO₂, from 4 to 10% of Al₂O₃, from 5to 15% of Na₂O, from 0 to 1% of K₂O, from 4 to 15% of MgO and from 0 to1% of ZrO₂, and has a total content SiO₂+Al₂O₃ of SiO₂ and Al₂O₃ of atmost 85%.

The present invention further provides the above glass for chemicaltempering, which contains at most 77% of SiO₂, at least 8% of Na₂O andfrom 4 to 14% of MgO, has a total content SiO₂+Al₂O₃ of at most 85%, hasa content of CaO of less than 1% if contained, and wherein R calculatedby the following formula by using the contents of the respectivecomponents is at least 0.66 (hereinafter this glass for chemicaltempering will sometimes be referred to as glass α of the presentinvention):

R=0.029×SiO₂+0.021×Al₂O₃+0.016×MgO−0.004×CaO+0.016×ZrO₂+0.029×Na₂O+0×K₂O−2.002

The present invention further provides the above glass for chemicaltempering, wherein D calculated by the following formula by using thecontents of the respective components is at most 0.18 (hereinafter thisglass for chemical tempering will sometimes be referred to as glass β ofthe present invention):

D=12.8−0.123×SiO₂−0.160×Al₂O₃−0.157×MgO−0.163×ZrO₂−0.113×Na₂O

The present invention further provides glass for chemical tempering,wherein when it is formed into a glass plate having a thickness of 1 mmand chemically tempered, and when a force of 98 N is applied to itsmirror-polished surface by means of a Vickers indenter, the probabilitythat the chemically tempered glass plate is broken is at most 10%.

Further, the present invention provides glass for chemical tempering,wherein when it is formed into a glass plate having a thickness of 1 mmand chemically tempered, and when a force of 196 N is applied to itsmirror-polished surface by means of a Knoop indenter, the probabilitythat the chemically tempered glass plate is broken is at most 10%(hereinafter this glass for chemical tempering will sometimes bereferred to as glass A).

The present invention further provides the above glass for chemicaltempering, wherein Δ represented by the following formula is at most0.21:

Δ=(S₄₀₀-S₄₅₀)/S₄₀₀

where S₄₀₀ is a surface compressive stress obtained when the glass isformed into a glass plate having a thickness of 1 mm and immersed inKNO₃ at 400° C. for 6 hours, and S₄₅₀ is a surface compressive stressobtained when the glass is formed into a glass plate having a thicknessof 1 mm and immersed in KNO₃ at 450° C. for 6 hours.

The present invention further provides chemically tempered glass, whichis obtained by chemically tempering the above glass for chemicaltempering.

The present invention further provides the above chemically temperedglass, which has a compressive stress layer thickness of at least 10 μmand a surface compressive stress of at least 400 MPa.

The present invention further provides a glass plate for a displaydevice, which is obtained by chemically tempering a glass platecomprising the above glass for chemical tempering.

The present invention further provides a display device, which has acover glass comprising the above glass plate for a display device.

The present invention still further provides the above display device,wherein the display device is a mobile device, a touch panel or a flatscreen television having a size of at least 20 inches.

Heretofore, the degree how chemically tempered glass is easily brokenhas been considered by means of the above S and t as indices, however,the present inventors have conducted researches employing the degree howcracking form when the chemically tempered glass itself hasindentations, and accomplished the present invention.

Advantageous Effects of Invention

According to the present invention, it is possible to obtain glass forchemical tempering, of which the strength can be sufficiently improvedby chemical tempering, and which is less likely to have crackingresulting from indentations made when the glass is used.

Further, it is possible to obtain chemically tempered glass which isless likely to be broken even when a load such as impact or a staticload is applied to the glass, since the strength of the glass is hardlydecreased even when indentations are made, and glass for chemicaltempering suitable for such chemically tempered glass.

Further, it is possible to obtain glass for chemical tempering, which isless likely to have cracking resulting from flaws before chemicaltempering treatment, latent scratches at the time of processing theglass and chipping, and which has a reduced possibility of spontaneousbreakage resulting from the cracking which occurs when the obtainablechemically tempered glass is used.

Further, it is possible to obtain a display device such as a mobiledevice, a touch panel or a flat screen television, using such glass forchemical tempering as a glass plate for a display device, such as acover glass.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing the relation between R obtained by calculationfrom the glass composition and the decrease ratio r of the surfacecompressive stress due to an increase of the Na concentration in themolten potassium salt.

FIG. 2 is a graph showing the relation between D obtained by calculationfrom the glass composition and the decrease ratio i.e. the stressrelaxation ratio Δ of the surface compressive stress obtainable whenglass is immersed in a molten potassium nitrate salt at 400° C. and at450° C. for 6 hours.

DESCRIPTION OF EMBODIMENTS

The chemically tempered glass and the glass plate for a display deviceof the present invention are obtained by chemically tempering the glassfor chemical tempering of the present invention, and hereinafter theywill generally be referred to as tempered glass of the presentinvention.

The above S of the tempered glass of the present invention is usually atleast 550 MPa, typically at least 650 MPa, when the tempered glass isused for e.g. a display device. Further, e.g. in a case where thethickness of the glass is lower than 2 mm, S is preferably at most 1,400MPa. If it exceeds 1,400 MPa, the internal tensile stress may be toogreat. It is more preferably at most 1,300 MPa, typically at most 1,200MPa.

The thickness t of the surface compressive stress layer of the temperedglass of the present invention preferably exceeds 10 μm, more preferablyexceeds 15 μm, typically exceeds 20 μm, when the tempered glass is usedfor e.g. a display device. Further, e.g. in a case where the thicknessof the tempered glass is lower than 2 mm, t is preferably at most 90 μm.If it exceeds 90 μm, the internal tensile stress may be too great. It ismore preferably at most 80 μm, typically at most 70 μm.

The method of chemical tempering treatment to obtain the tempered glassof the present invention is not particularly limited so long as Na ionsin the glass surface layer can be ion exchanged with K ions in themolten salt, and it may, for example, be a method of immersing the glassin a heated potassium nitrate (KNO₃) molten salt.

Chemical tempering treatment conditions to form a chemically temperedlayer (compressive stress layer) having a desired surface compressivestress on the glass may vary depending upon e.g. the thickness in thecase of a glass plate. However, it is typical to immerse a glasssubstrate in a KNO₃ molten salt at from 350 to 550° C. for from 2 to 20hours. From the economical viewpoint, the immersion is carried out underconditions of from 350 to 500° C. and from 2 to 16 hours, and morepreferably, the immersion time is from 2 to 10 hours.

The tempered glass of the present invention, particularly the glassplate for a display device of the present invention, is preferably notbroken even when a force of 5 kgf=49 N is applied by means of a Vickersindenter of a Vickers hardness meter. It is more preferably not brokeneven when a force of 7 kgf is applied, particularly preferably notbroken even when a force of 10 kgf is applied. Further, the breakageratio when a force of 20 kgf=196 N is applied by means of a Vickersindenter is preferably at most 20%, more preferably at most 10%.

Further, the tempered glass of the present invention, particularly theglass plate for a display device of the present invention, is preferablynot broken even when a force of 10 kgf=98 N is applied by means of aKnoop indenter of a Knoop hardness meter. It is more preferred that thebreakage ratio when a force of 20 kgf is applied is at most 10%, and itis particularly preferred that the breakage ratio when a force of 30 kgfis applied is at most 10%. The breakage ratio when a force of 20 kgf isapplied to one obtained by chemically tempering the glass A is at most10%.

The glass plate for a display device of the present invention is usuallyobtained by chemically tempering a glass plate obtained by processing aglass plate made of the glass for chemical tempering of the presentinvention by e.g. cutting, hole making, polishing, etc.

The thickness of the glass plate for a display device of the presentinvention is usually from 0.3 to 2 mm, typically at most 1.5 mm.

The glass plate for a display device of the present invention istypically a cover glass.

A method for producing a glass plate made of the above glass forchemical tempering is not particularly limited, and for example, variousraw materials are mixed in proper amounts, heated and melted at fromabout 1,400 to 1,700° C. and then homogenized by deforming, stirring orthe like and formed into a plate by a well-known float process, downdrawmethod or press method, which is annealed and then cut into a desiredsize to obtain the glass plate.

The glass transition point Tg of the glass for chemical tempering of thepresent invention, i.e. the glass of the present invention, ispreferably at least 400° C. If it is lower than 400° C., the surfacecompressive stress is likely to be relaxed during the ion exchange, andno adequate stress may be obtained.

The temperature T2 at which the viscosity of the glass of the presentinvention becomes 10² dPa·s is preferably at most 1,750° C.

The temperature T4 at which the viscosity of the glass of the presentinvention becomes 10⁴ dPa·s is preferably at most 1,350° C.

The specific gravity p of the glass of the present invention ispreferably at most 2.50.

The Young's modulus E of the glass of the present invention ispreferably at least 68 GPa. If it is less than 68 GPa, the crackingresistance or the breaking strength of the glass may be inadequate.

The Poisson's ratio a of the glass of the present invention ispreferably at most 0.25. If it exceeds 0.25, the cracking resistance ofthe glass may be inadequate.

Now, the glass α of the present invention will be described below.

As described above, usually, the ion exchange treatment for chemicaltempering is carried out by immersing glass containing sodium (Na) in amolten potassium salt, and as the potassium salt, potassium nitrate or amixed salt of potassium nitrate and sodium nitrate is used.

In such ion exchange treatment, ion exchange of Na in the glass withpotassium (K) in the molten salt is carried out. Therefore, if the ionexchange treatment is repeated by using the same molten salt, the Naconcentration in the molten salt increases.

If the Na concentration in the molten salt increases, the surfacecompressive stress S of the chemically tempered glass decreases, andtherefore, there has been a problem that it is necessary to strictlywatch the Na concentration in the molten salt and to frequently carryout replacement of the molten salt, so that S of the chemically temperedglass will not become lower than the desired value.

It is desired to reduce the frequency of such replacement of the moltensalt, and the glass α of the present invention is one of embodiments ofthe present invention suitable to solve such problems.

The present inventors have considered that there may be a relationbetween the composition of Na-containing glass and such a phenomenonthat by repeating ion exchange treatment of immersing the Na-containingglass in a molten potassium salt many times to obtain chemicallytempered glass, the Na concentration in the molten potassium saltincreases and at the same time, the surface compressive stress of thechemically tempered glass becomes small, and have conducted thefollowing experiment.

Firstly, 29 types of glass plates were prepared which had compositionsas represented by mole percentage in Tables 1 to 3 and each of which hada thickness of 1.5 mm and a size of 20 mm×20 mm and had both sidesmirror-polished with cerium oxide. The glass transition points Tg (unit:° C.) of these glasses are shown in the same Tables. Here, thoseprovided with * are ones calculated from the compositions.

These 29 types of glass plates were subjected to ion exchange ofimmersing for 10 hours in a molten potassium salt having a KNO₃ contentof 100% and having a temperature of 400° C. to obtain chemicallytempered glass plates, whereupon their surface compressive stresses CS1(unit: MPa) were measured. Here, glass A27 is glass used for a coverglass for a mobile device.

Further, these 29 types of glass plates were subjected to ion exchangeof immersing for 10 hours in a molten potassium salt having a KNO₃content of 95% and a NaNO₃ content of 5% and having a temperature of400° C. to obtain chemically tempered glass plates, whereupon theirsurface compressive stresses CS2 (unit: MPa) were measured.

CS1 and CS2 are shown together with their ratio r=CS2/CS1 in thecorresponding rows in Tables 1 to 3. r of conventional cover glass A27is 0.65.

TABLE 1 Glass α1 α2 A1 A2 A3 A4 A5 A6 A7 A8 SiO₂ 73.0 72.0 64.3 64.364.3 64.3 63.8 63.8 64.3 64.3 Al₂O₃ 7.0 6.0 6.5 7.0 6.5 7.0 7.0 7.5 6.06.0 MgO 6.0 10.0 11.0 11.0 11.0 11.0 11.0 11.0 11.5 12.0 CaO 0 0 0.1 0.10.1 0.1 0.1 0.1 0.1 0.1 SrO 0 0 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 BaO 0 00.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 ZrO₂ 0 0 2.0 1.5 1.5 1.0 1.5 1.0 2.0 1.5Na₂O 14.0 12.0 12.0 12.0 12.5 12.5 12.5 12.5 12.0 12.0 K₂O 0 0 4.0 4.04.0 4.0 4.0 4.0 4.0 4.0 Tg 617 647 615 617 608 603 614 610 615 609 CS1888 900 1049 1063 1035 1047 1063 1046 1020 1017 CS2 701 671 589 593 601590 601 599 588 579 r 0.79 0.75 0.56 0.56 0.58 0.56 0.57 0.57 0.58 0.57R 0.76 0.72 0.55 0.56 0.56 0.56 0.56 0.56 0.55 0.55

TABLE 2 Glass A9 A10 A11 A12 A13 A14 A15 A16 A17 A18 SiO₂  64.3  64.3 64.3  64.3  64.3  65.3  64.3  60.3  56.3  64.3 Al₂O₃   7.2   7.0  6.0 6.0  8.0  7.0  10.0  11.5  15.5   8.0 MgO  11.0  11.0  12.5  13.0  11.0 11.0   8.5  11.0  11.0  10.5 CaO   0.1   0.1  0.1  0.1  0.1  0.1   0.1  0.1   0.1   0.1 SrO   0.1   0.1  0.1  0.1  0.1  0.1   0.1   0.1   0.1  0.1 BaO   0.1   0.1  0.1  0.1  0.1  0.1   0.1   0.1   0.1   0.1 ZrO₂  0.5   1.5  1.0  0.5  0.5  0.5   0   0   0   0.5 Na₂O  12.7  11.5  12.0 12.0  12.0  12.0  13.0  13.0  13.0  12.5 K₂O   4.0   4.5  4.0  4.0  4.0 4.0   4.0   4.0   4.0   4.0 Tg  597  612* 610* 610* 614 610*  620* 630*  670*  608 CS1 1003 1013 984 963 954 983 1072 1145 1221 1024 CS2 588  564 561 546 576 574  640  641  647  582 r   0.59   0.56  0.57 0.57  0.60  0.58   0.60   0.56   0.53   0.57 R   0.57   0.54  0.55 0.55  0.56  0.57   0.59   0.54   0.51   0.57

TABLE 3 Glass A19 A20 A21 A22 A23 A24 A25 A26 A27 SiO₂  64.3  63.5  66.0 64.5  65.0  63.5  64.3  71.3  66.7 Al₂O₃  8.5  10.5   9.0  9.0  5.0 5.0   6.0  2.0  10.8 MgO  10.5   9.0   8.0  12.0  12.0  8.0  11.0  10.4  6.2 CaO  0.1   0   0  0  0.5  4.0   0.1  0.3   0.6 SrO  0.1   0   0  0 0  0   0.1  0.03   0 BaO  0.1   0   0  0  0  0   0.1  0.02   0 ZrO₂  0  0   0  0  0  1.3   2.5  0.5   0 Na₂O  12.5  15.0  15.0  11.5  11.0 9.4  12.0  10.8  13.2 K₂O  4.0   2.0   2.0  3.0  6.5  8.9   4.0  4.6  2.4 Tg 610*  630*  610* 650* 570* 580*  620 580*  595 CS1 985 11901054 919 746 668 1019 664 1039 CS2 577  752  722 516 382 240  571 407 679 r  0.59   0.63   0.69  0.56  0.51  0.36   0.56  0.61   0.65 R  0.57  0.64   0.66  0.58  0.50  0.35   0.55  0.59   0.64

From these results, it has been found that there is a high correlationbetween R calculated by the above formula (shown in the bottom rows inTables 1 to 3) and the above r. FIG. 1 is a scatter graph to make thispoint clear wherein the abscissa represents R and the ordinaterepresents r, and the straight line in the Fig. representsr=1.027×R-0.0017. The correlation coefficient is 0.97.

From the above correlation found by the present inventors, the followingis evident. That is, in order to reduce the frequency of replacement ofthe molten salt, glass having a less degree of decrease in S due to anincrease of the Na concentration in the molten salt i.e. glass havingthe above r being large, may be used, and for such a purpose, the aboveR of the glass may be made to be large.

The glass α of the present invention is achieved by such discoveries.

By making R being at least 0.66, the above r can be made to be at least0.66 and as a result, it is possible to ease controls of the Naconcentration in the molten salt as compared with conventional one, orit is possible to reduce the frequency of replacement of the moltensalt. R is preferably at least 0.68.

Further, when glasses α1 and α2 having r being largest among 29 types ofglasses, are compared with the other 27 types of glasses, they arecommon in that they contain no K₂O. On the other hand, the coefficientrelating to K₂O in the above formula for calculation of R is 0 and issubstantially small as compared with the coefficient of 0.029 relatingto Na₂O being the same alkali metal oxide, and this explains such apoint.

Accordingly, from such a viewpoint, the glass of the present inventionpreferably contains no K₂O, and as such glass, the following glass maybe mentioned. That is, it is glass for chemical tempering whichcomprises, as represented by mole percentage based on the followingoxides, at most 77% of SiO₂, at least 3% of MgO, from 0 and less than 1%of CaO and at least 8% of Na₂O, has a total content of SiO₂ and Al₂O₃ ofat most 85%, and contains no K₂O.

Now, the composition of the glass of the present invention will bedescribed by using contents represented by mole percentage unlessotherwise specified.

SiO₂ is a component to constitute a glass matrix and is essential, andis a component to reduce formation of cracking when the glass surfacehas flaws (indentations) or to reduce the breakage ratio whenindentations are impressed after chemical tempering. If the SiO₂ contentis less than 65%, stability or weather resistance of the glass or thechipping resistance tends to decrease, and when the SiO₂ content is atleast 65%, a change in the surface compressive stress due to a NaNO₃concentration in the KNO₃ molten salt can be made small. The SiO₂content is preferably at least 68%, more preferably at least 70%. If theSiO₂ content exceeds 85%, the viscosity of the glass tends to increasethereby to lower the melting property, and it is preferably at most 80%,more preferably at most 77%, particularly preferably at most 75%. In theglass a of the present invention, the SiO₂ content is considered to beat most 77%, and is preferably at most 76%, more preferably at most 75%.

Al₂O₃ is a component to improve the ion exchange performance and thechipping resistance, or to reduce the breakage ratio when indentationsare impressed after chemical tempering. If the Al₂O₃ content is lessthan 3%, no desired surface compressive stress or the compressive stresslayer thickness will be obtained by ion exchange. It is preferably atleast 4%, more preferably at least 4.5%, particularly preferably atleast 5%. If the Al₂O₃ content exceeds 15%, the viscosity of the glasstends to be high, whereby homogeneous melting tends to be difficult.Further, when the Al₂O₃ content is at most 15%, a change in the surfacecompressive stress due to a NaNO₃ concentration in the KNO₃ molten saltcan be made small. The Al₂O₃ content is preferably at most 12%, morepreferably at most 11%, further preferably at most 10%, particularlypreferably at most 9%, typically at most 8%. In a case where a stressrelaxation when chemical tempering treatment is carried out is to besuppressed, the Al₂O₃ content is preferably at most 6%. If the Al₂O₃content exceeds 6%, it is necessary that a larger amount of Na₂O iscontained to maintain the melting property of the glass and as a result,the above stress relaxation is likely to occur.

If the total content SiO₂+Al₂O₃ of SiO₂ and Al₂O₃ exceeds 88%, theviscosity of the glass at high temperature tends to increase and themelting tends to be difficult, and it is preferably at most 85%, morepreferably at most 83%. Further, the total content SiP₂+Al₂O₃ ispreferably at least 75%. If the total content SiO₂+Al₂O₃ is less than75%, the crack resistance when the glass surface has indentations tendsto decrease, and it is more preferably at least 77%.

Na₂O is a component to form a surface compressive stress layer by ionexchange and to improve the melting property of the glass, and isessential. If the Na₂O content is less than 5%, it tends to be difficultto form a desired surface compressive stress layer by ion exchange, andit is preferably at least 8%. In a case where a change in the surfacecompressive stress due to a NaNO₃ concentration in the KNO₃ molten saltis to be made small, the Na₂O content is preferably at least 8%, and inthe glass α of the present invention, the Na₂O content is considered tobe at least 8%, and is preferably at least 9%, more preferably at least10%, further preferably at least 11%, particularly preferably at least12%. If the Na₂O content exceeds 15%, the weather resistance tends todecrease, or cracking is likely to be formed from an indentation.

K₂O is not essential but is a component to increase the ion exchangerate, and thus, it may be contained in a range of less than 2%. If itscontent is at least 2%, cracking is likely to be formed from anindentation, or the change in the surface compressive stress due to aNaNO₃ concentration in the KNO₃ molten salt becomes large. The K₂Ocontent is preferably at most 1.9%, more preferably at most 1%,typically at most 0.8%. As described above, in a case where the changein the surface compressive stress due to a NaNO₃ concentration in theKNO₃ molten salt is to be made small, it is preferred that no K₂O iscontained.

MgO is a component which may decrease the ion exchange rate and is notessential, but is a component to suppress cracking and to improve themelting property, and may be contained up to 15%. If the MgO content isless than 3%, the viscosity tends to increase and it is highly possiblethat the melting property decreases, and from such a viewpoint, MgO iscontained preferably in a content of at least 3%, more preferably atleast 4%, particularly preferably at least 5%. In the glass α of thepresent invention, the MgO content is considered to be at least 3%. In acase where the stress relaxation is to be suppressed, the MgO content ispreferably at least 8%. If the MgO content is less than 8%, the degreeof the stress relaxation tends to vary depending upon the location in achemical tempering treatment tank due to dispersion of the molten salttemperature when chemical tempering treatment is carried out and as aresult, it may be difficult to obtain a stable compressive stress. Ifthe MgO content exceeds 15%, the glass is likely to devitrify, or thechange in the surface compressive stress due to a NaNO₃ concentration inthe KNO₃ molten salt tends to be large, and the MgO content ispreferably at most 12%. In the glass α of the present invention, the MgOcontent is more preferably at most 11%, further preferably at most 10%,particularly preferably at most 8%, typically at most 7%.

The total content of SiO₂, Al₂O₃, Na₂O and MgO is preferably at least98%. If the total content is less than 98%, it may be difficult toobtain a desired compressive stress layer while maintaining the chippingresistance. It is typically at least 98.3%.

ZrO₂ is not essential, but may be contained up to 1% so as to lower theviscosity at high temperature or to increase the surface compressivestress. If the ZrO₂ content exceeds 1%, cracking is likely to be formedfrom an indentation.

In a case where the SiO₂ content is at most 72%, the ZrO₂ content ispreferably at most 0.63%. If the ZrO₂ content exceeds 0.63%, the glassis likely to be broken when it gets an indentation after chemicaltempering, and from such a viewpoint, it is more preferred that no ZrO₂is contained in such a case.

The glass of the present invention essentially comprises theabove-described components, but may contain other components within arange not to impair the object of the present invention. In a case wheresuch other components are contained, the total content of suchcomponents is preferably at most 5%, more preferably at most 3%,typically at most 1%. Now, such other components will be exemplified.

ZnO may be contained up to 2% for example in some cases, in order toimprove the melting property of glass at a high temperature, but itscontent is preferably at most 1%, and preferably at most 0.5% in a caseof production by a float process. If the ZnO content exceeds 0.5%, it islikely to be reduced during the float forming to form a product defect.Typically no ZnO is contained.

B₂O₃ may be contained within a range less than 1% for example in somecases, in order to improve the melting property of glass at a hightemperature. If the B₂O₃ content is at least 1%, homogeneous glass tendsto be hardly obtainable, and the glass forming may be difficult, or thechipping resistance may deteriorate. Typically no B₂O₃ is contained.

TiO₂ is likely to deteriorate the visible light transmittance and tocolor glass to be brown when it is coexistent with Fe ions in the glass,and therefore, its content is preferably at most 1% if contained, andtypically, it is not contained.

Li₂O is a component to lower the strain point and to bring about astress relaxation thereby to make it difficult to stably obtain asurface compressive stress layer and therefore it is preferably notcontained, and even if contained, its content is preferably less than1%, more preferably at most 0.05%, particularly preferably less than0.01%.

Further, Li₂O may elute in a molten salt of e.g. KNO₃ at the time of thechemical tempering treatment in some cases, and if the chemicaltempering treatment is carried out by using a molten salt containing Li,the surface compressive stress remarkably decrease. That is, the presentinventors have found that when the glass in Example 23 describedhereinafter was subjected to a chemical tempering treatment at 450° C.for 6 hours by using KNO₃ containing no Li and KNO₃ containing Li incontents of 0.005 mass %, 0.01 mass % and 0.04 mass %, the surfacecompressive stress remarkably decreased only when the molten saltcontained 0.005 mass % of Li. That is, Li₂O is preferably not containedfrom such a viewpoint.

CaO may be contained within a range less than 1% in order to improve themelting property at a high temperature or to prevent devitrification. Ifthe CaO content is at least 1%, the ion exchange rate or the durabilityagainst cracking tends to decrease. Typically no CaO is contained.

SrO may be contained as the case requires, but it has a higher effect oflowering the ion exchange rate as compared with MgO and CaO, andtherefore its content is preferably less than 1% even if contained.Typically no SrO is contained.

BaO has the highest effect of lowering the ion exchange rate amongalkaline earth metal oxides, and therefore it is preferred that no BaOis contained or even if contained, its content is less than 1%.

In a case where SrO or BaO is contained, their total content ispreferably at most 1%, more preferably less than 0.3%.

In a case where at least one of CaO, SrO, BaO and ZrO₂ is contained, thetotal content of these four components is preferably less than 1.5%. Ifthe total content is at least 1.5%, the ion exchange rate may decrease,and it is typically at most 1%.

As a clarifying agent at the time of melting glass, SO₃, a chloride, afluoride or the like may suitably be contained. However, in order toincrease the visibility of display devices such as touch panels, it ispreferred to reduce contamination by impurities such as Fe₂O₃, NiO orCr₂O₃ having an absorption in a visible light range in raw materials asfar as possible, and the content of each of them is preferably at most0.15%, more preferably at most 0.05%, as represented by mass percentage.

Further, the glass for chemical tempering is preferably one which can bechemically tempered in a short time, and if it is attempted to carry outchemical tempering in a short time, it is necessary to increase the ionexchange temperature i.e. the temperature of the molten salt so as toincrease the ion exchange rate. However, if the ion exchange temperatureis increased, the surface compressive stress S formed by the chemicaltempering is likely to decrease. Hereinafter in the present invention,this phenomenon is called a stress relaxation, and the above Δ which isan index of the stress relaxation is preferably at most 0.21 when theimportance is placed on the stability of S. That is, the dispersion of Sis required to be within 5%, and on the other hand, the fluctuation ofthe temperature of the molten salt in a chemical tempering treatmenttank is ±6° C. and the fluctuation full width i.e. the dispersion is12%, and therefore when A evaluated by a temperature difference of 50°C. (=450° C-400° C.) is at most 0.21, the dispersion of S at 12° C. is0.05=5% which is 12/50 thereof. Δ is more/ preferably at most 0.20,particularly preferably at most 0.19.

The present inventors have further found that Δ varies depending uponthe glass composition, and achieved glass β of the present invention.FIG. 2 is a scatter graph illustrating the relation between the above Dcalculated from the compositions of glasses in Examples 1 to 12, 31 to43, 57 and 59 to 62 described hereinafter and Δ of the respectiveglasses. The straight line in the Fig. represents D=0.911×Δ-0.018, andthe correlation coefficient is 0.91. That is, when D is at most 0.18, itis possible that Δ is substantially at most 0.21. D is preferably atmost 0.17, more preferably at most 0.16.

In a case where Δ is to be made small, the glass of the presentinvention preferably comprises, as represented by mole percentage, from70 to 75% of SiO₂, from 5.5 to 8.5% of Al₂O₃, from 12 to 15% of Na₂O,from 0 to 1% of K₂O, higher than 7% and at most 9% of MgO and from 0 to0.5% of ZrO₂, and has a total content SiO₂+Al₂O₃ of SiO₂ and Al₂O₃ of atmost 83%. CaO is a component which is likely to inhibit the ion exchangeand make it difficult to obtain a sufficient t, and is a component whichpromotes cracking when an indenter is pressed, and from such viewpoints,it is preferably not contained, and even if contained, its content ispreferably less than 1%.

EXAMPLES

In Examples 1 to 16, 23 to 26, 28, 29 and 31 to 62 in Tables 4 to 11,glass raw materials which are commonly employed, such as oxides,hydroxides, carbonates and nitrates, were properly selected to havecompositions as represented by mole percentage in columns for SiO₂ toK₂O and weighed to obtain 400 g of glass. To these weighed glass rawmaterials, sodium sulfate in a mass corresponding to 0.2% of their masswas added, and they were mixed. Then, the mixed raw materials were putin a platinum crucible, the platinum crucible was put in a resistanceheat type electric furnace at 1,650° C., and the mixture was melted for5 hours and refined and homogenized. The obtained molten glass was castinto a mold, held for one hour at a temperature of Tg+50° C. and thencooled to room temperature at a rate of 0.5° C./min to obtain a glassblock. This glass block was cut and polished and finally their bothsurfaces were mirror-polished to obtain a plate glass having a size of30 mm×30 mm and a thickness of 1.0 mm.

Example 27 in Table 7 corresponds to soda lime glass separatelyprepared, and in Examples 17 and 18 in Table 5, Examples 19 to 22 inTable 6 and Example 30 in Table 7, no melting of glass and the like asdescribed above were conducted. Examples 1 to 22 and 30 to 62 areExamples of the present invention, and Examples 23 to 29 are ComparativeExamples.

For reference, the compositions of glasses in Examples 1 to 62 asrepresented by mass percentage are shown in Tables 12 to 19.

With respect to these glasses, the glass transition point Tg (unit: °C.), the temperature T2 (unit: ° C.) at which the viscosity becomes 10²dPa·s, the temperature T4 (unit: ° C.) at which the viscosity becomes10⁴ dPa·s, the specific gravity p, the average linear expansioncoefficient α (unit: ⁻⁷/° C.) at from 50 to 350° C., the Young's modulusE (unit: GPa), the Poisson's ratio a, the cracking probability P₀ (unit:%) when not tempered, the above r, the above R, the above Δ and theabove D are shown in Tables. Data with * in Tables are values obtainedby calculation or assumption from the compositions.

P₀ is a cracking probability when a load of 500 gf (=4.9 N) was appliedby using a Vickers hardness meter and measured as follows.

A plate glass was ground by means of a #1000 grinder for from 300 to1,000 μm to obtain a plate glass, which was polished by means of ceriumoxide to obtain mirror surfaces. Then, in order to remove the processingstrain on the mirror surfaces, the plate glass was heated to atemperature of Tg+50° C. in a resistance heat type electric furnaceunder the atmospheric pressure, held at this temperature for one hourand then cooled to room temperature at a rate of 0.5° C./min. Theheating was conducted at a heating rate such that the temperatureachieved Tg in one hour.

Using the above treated sample, the cracking probability was measured.That is, a Vickers indenter was pressed at ten points with a load of aVickers hardness meter of 500 g in the air atmosphere at a temperatureof from 20 to 28° C. at a dew point of −30° C., and the number of cracksformed at four corners of indentations was measured. The number of suchcracks was divided by the possible number of cracks being 40 to obtainthe cracking probability.

The cracking probability of glass when not tempered is preferably lower.

Specifically, P₀ is preferably at most 50%. With respect to the glassesin Examples of the present invention, P₀ does not exceed 50% in eachExample, and it is found that cracking is less likely to form even in anon-tempered state.

Then, the plate glasses in Examples 1 to 16, 23 to 29 and 59 to 62 weresubjected to the following chemical tempering treatment. That is, eachglass was immersed in a KNO₃ molten salt at 400° C. for 8 hours to carryout a chemical tempering treatment. In the KNO₃ molten salt, the KNO₃content was from 99.7 to 100%, and the NaNO₃ content was from 0 to 0.3%.

With respect to each glass after the chemical tempering treatment, thesurface compressive stress S (unit: MPa) and the compressive stresslayer depth t (unit: μm) were measured by means of a surface stressmeter FSM-6000 manufactured by Orihara Manufacturing Co., Ltd. Theresults are shown in the corresponding rows in Tables.

Further, with respect to the plate glasses in Examples 31 to 58, thesurface compressive stress and the compressive stress layer depth weremeasured in the same manner with times of immersion in the KNO₃ moltensalt at 400° C. of 6 hours and 10 hours, and from these values, thesurface compressive stress and the compressive stress layer depth whenthe above immersion time was 8 hours were estimated. The results areshown in columns for S and t in the Tables.

Further, with respect to 20 sheets each of the plate glasses after thechemical tempering treatment in Examples 1 to 18 and 23 to 29 and 20sheets of the plate glasses after the chemical tempering treatment for10 hours (the above chemical tempering treatment wherein the time ofimmersion in the KNO₃ molten salt at 400° C. was 10 hours) in Examples31 to 62, a Vickers indenter of a Vickers hardness meter was pressedunder 5 kgf i.e. 49 N in the atmospheric pressure at a temperature offrom 20 to 28° C. under a humidity of from 40 to 60%, and the number ofbreakage originating from the indentation was divided by the number ofthe plates measured being 20 and represented by percentage, to obtainthe breakage ratio P₁ (unit: %). Further, the breakage ratio P₂ (unit:%) differing from P₁ only in that the Vickers indenter was pressed under10 kgf i.e. 98 N was measured in the same manner as P₁. P₁ and P₂ arerespectively preferably at most 50% and at most 40%.

In Examples 1 to 15, 17, 18 and 31 to 62 of the present invention, theglasses were not broken at all and P₁ was 0%, and even in Example 16 inwhich P₁ was not 0%, both P₁ and P₂ were only 40%, whereas inComparative Examples 24 to 29, P₁ or P₂ exceeded 40%, and particularlywith respect to the glasses in Examples 27 to 29, both P₁ and P₂ were100% and all the glasses were broken. That is, it is found that theglass of the present invention has a low risk of breakage even when ithas an indentation. In Comparative Examples 23 and 26, P₁ was at most40%, but P₀ was so high as exceeding 50% in these Comparative Examples.

Further, with respect to the glasses in Examples 1, 8 and 27 to 29,glasses having a size of 5 mm×40 mm×1 mm thickness and having thesurfaces of 5 mm×40 mm mirror-polished and having the other sidesprocessed to #1000 finish, were separately prepared. These glasses weresubjected to a chemical tempering treatment at from 425 to 450° C. byusing a potassium nitrate molten salt (KNO₃: 98 to 99.8%, NaNO₃: 0.2 to2%). The surface compressive stress and the compressive stress layerdepth were 757 MPa and 55 μm in Example 1, 878 MPa and 52 μm in Example8, 607 MPa and 15 μm in Example 27, 790 MPa and 49 μm in Example 28, and830 MPa and 59 μm in Example 29, respectively.

On the center of the mirror-polished surface of 5 mm×40 mm of each ofthe glasses after the chemical tempering treatment, a Vickers indenterwas pressed under a load of 10 kgf by using a Vickers hardness meter toform an indentation. The glasses in Comparative Examples 27 to 29 werebroken when the indentation was formed, but the glasses in Examples 1and 8 were not broken.

By using the samples in Examples 1 and 8 with an indentation of 10 kgf,a three point bending test was carried out with a span of 30 mm so thatthe surface with the indentation was pulled. The average bendingstrength (unit: MPa) with n=20 is shown in the column for F in Table 4.Chemically tempered glasses in Examples 1 and 8 showed a very highbreaking stress of at least 400 MPa even in a state where theindentation was formed.

A was measured as follows. That is, ion exchange of immersing the glassin molten potassium nitrate having a KNO₃ content of 100% and atemperature of 400° C. for 6 hours was carried out to prepare achemically tempered glass plate, and the surface compressive stress S₄₀₀(unit: MPa) was measured. Further, ion exchange of immersing the glassin molten potassium nitrate having a KNO₃ content of 100% and atemperature of 450° C. for 6 hours was carried out to prepare achemically tempered glass plate, and the surface compressive stress S₄₅₀(unit: MPa) was measured. From S₄₀₀ and ₅₄₅₀ thus measured,(S₄₀₀-S₄₅₀)/S₄₀₀ was calculated and regarded as A.

TABLE 4 Ex. 1 2 3 4 5 6 7 8 9 SiO₂  73.0  75.5  73.0  73.0  73.0  73.0 73.2  72.0  72.0 Al₂O₃   7.0   4.9   5.0   5.0   7.0   7.0   7.0   6.0  7.0 MgO   6.0   5.9   8.0  10.0   5.5   5.5   5.5  10.0  10.0 CaO   0  0   0   0   0   0   0   0   0 ZrO₂   0   0   0   0   0.5   0.5   0.3  0   0 Na₂O  14.0  13.7  14.0  12.0  14.0  14.0  14.0  12.0  11.0 K₂O  0   0   0   0   0   0   0   0   0 Tg  617  586  600  632  625  617 620  647  674 T2 1734 1680* 1642* 1652* 1696* 1721* 1710* 1711 1687* T41256 1195* 1170* 1187* 1214* 1206* 1203* 1256 1225* ρ   2.405   2.392  2.408   2.410   2.417   2.409   2.424   2.412   2.409 α  79  78  80 72  77.25  77.26  77.74  71.71  68 E  70.8  69.7  70.6  72.9  73  72.3 74.6  73.1  72.8 σ   0.204   0.203   0.207   0.207   0.226   0.23  0.218   0.207   0.226 P₀  10  10   0   0  10   0   5   0   7.5 S  909 699  821  918  931  864  878  943  915 t  33  34  30  23  33  35  33 24  23 P₁   0   0   0   0   0   0   0   0   0 P₂   0   0   0   0   0  0   0   0   0 F  495  445*  505*  552*  559*  526*  533*  565  550* r  0.79   0.84   0.80   0.71   0.79   0.80   0.79   0.75   0.71 R   0.76  0.78   0.75   0.73   0.76   0.76   0.77   0.72   0.71 Δ   0.196  0.294   0.253   0.154   0.186   0.228   0.222   0.082   0.017 D  0.177   0.256   0.183   0.095   0.256   0.256   0.224   0.058   0.011

TABLE 5 Ex. 10 11 12 13 14 15 16 17 18 SiO₂  72.0  72.0  72.0  71.7 71.4  70.0  70.1  73.0  77.9 Al₂O₃   7.0   6.0   6.0   7.1   8.2   9.0  6.0   9.0   4.8 MgO   9.0  12.0  14.0   8.1   6.1   7.0  10.3   6.0  5.8 CaO   0   0   0   0   0   0   0   0   0 ZrO₂   0   0   0   0   0  0   0.63   0   0 Na₂O  12.0  10.0   8.0  13.1  14.3  14.0  12.0  12.0 11.5 K₂O   0   0   0   0   0   0   1.0   0   0 Tg  660  678  701  635* 629*  640*  634*  612*  628* T2 1682* 1669* 1679* 1674* 1689* 1687*1638* 1744* 1727* T4 1216* 1214* 1231* 1205* 1214* 1219* 1187* 1271*1236* ρ   2.410   2.415   2.419   2.41*   2.41*   2.42*   2.44*   2.40*  2.37* α  72  65  57  78*  81*  81*  79*  72*  70* E  72.3  74.3  73.3 73*  73*  74*  74*  71*  69* σ   0.23   0.218   0.207   0.20*   0.20*  0.20*   0.20*   0.20*   0.20* P₀  15  10  15  22.5  32.5  35  25   0*  0* S  974  853  667  983  970 1101  928 1069*  749* t  25  18  15  31 36  33  27  17*  27* P₁   0   0   0   0   0   0  40   0   0 P₂   0   0  0   0   0   0  40   0   0 F  579*  520*  430*  584*  578*  641*  557* 626*  470* r   0.74   0.69   0.74   0.75   0.77   0.74   0.70 — — R  0.73   0.69   0.67   0.74   0.75   0.74   0.68   0.75   0.78 Δ   0.099  0.065  −0.090 D   0.055  −0.030  −0.118   0.095   0.133   0.069  0.249   0.083   0.241

TABLE 6 Ex. 19 20 21 22 SiO₂  65.0  70.1 84  80  Al₂O₃  15.0 5 3 4 MgO 5.0 15  1 9 CaO 0 0 0 0 ZrO₂ 0 1 0 0 Na₂O 15  7 12  6 K₂O 0  1.9 0 1 Tg647*  665*  571*  650*  T2 1748*  1650*  1785*  1617*  T4 1279*  1215* 1258*  1780*  ρ   2.44*   2.45*   2.33*   2.35* α 83* 66* 79* 54* E 74*79* 66* 73* σ   0.20*   0.20*   0.17*   0.18* P₀ 10* 20*  0*  0* S1200*  960*  1142*  1037*  t 34* 30* 28* 20* P₁  0*  0*  0*  0* P₂  0* 0*  0*  0* F 690*  573*  662*  611*  r — — — — R   0.71   0.59   0.86  0.72 Δ — — — — D   −0.075   0.232   0.475   0.229

TABLE 7 Ex. 23 24 25 26 27 28 29 30 SiO₂  71.1  68.3  66.4  66.0  72.0 64.5  66.6  77.0 Al₂O₃   9.3   6.0   6.0   7.0   1.1   6.0  10.8   3.0MgO   4.1  10.5  10.8  11.0   5.5  11.0   6.2  12.0 CaO   0   0   0   0  8.6   0   0.6   0 ZrO₂   0   1.3   1.9   0   0   2.5   0   0 Na₂O 15.5  12.0  12.0  12.0  12.6  12.0  13.2   8 K₂O   0   2.0   3.0   4.0  0.2   4.0   2.4   0 Tg  623*  629*  623*  596*  540  620  590  660* T21704* 1614* 1592* 1609* 1461 1575 1686* 1688* T4 1223* 1178* 1168* 1170*1039 1168 1240* 1221* ρ   2.41*   2.47*   2.50*   2.46*   2.49   2.53  2.46   2.38* α  84*  83*  87*  93*  87  91  93  60* E  73*  74*  75* 74*  73  78  73*  76* σ   0.20*   0.20*   0.20*   0.20*   0.20*   0.22  0.21*   0.21* P₀  70  60  82.5  82.5  90  82.5  60   0* S  943  974 894  831  713  987  843  985* t  40  29  27  37  10  34  37  25* P₁   0 60  90  18  100  100  100   0* P₂   0  100  100  60  100  100  100   0*F — — — — — — —  585* r   0.80   0.66   0.61   0.62 — — — — R   0.77  0.64   0.60   0.58   0.53   0.56   0.64   0.72 Δ   0.330   0.038  0.119 D   0.171   0.441   0.632   0.479   1.481   0.824   0.410  0.061

TABLE 8 Ex. 31 32 33 34 35 36 37 38 39 SiO₂  73.6  72.4  74.0  72.0 73.6  72.4  73.7  72.3  73.0 Al₂O₃   6.5   7.5   7.0   7.0   7.0   7.0  8.1   5.9   8.0 MgO   6.0   6.0   5.0   7.0   6.0   6.0   4.0   7.9  6.0 CaO   0   0   0   0   0   0   0   0   0 ZrO₂   0   0   0   0   0  0   0   0   0 Na₂O  13.9  14.1  14.0  14.0  13.4  14.6  14.1  13.9 13.0 K₂O   0   0   0   0   0   0   0   0   0 Tg  613  628  613  623 626  603  625  612  654 T2 1686* 1690* 1704* 1672* 1700.* 1676* 1724*1653* 1716* T4 1206* 1213* 1220* 1199* 1220* 1199* 1237* 1182* 1238* ρ  2.40*   2.408   2.39*   2.41   2.399   2.41*   2.39*   2.41*   2.399 α 80*  80*  79*  81*  78*  82*  79*  81*  76* E  71*  69.3  71*  69.7 69.3  72*  70*  73*  70 σ   0.2*   0.2   0.2*   0.2   0.2   0.2*   0.2*  0.2*   0.2 P₀   0*   0*   0*   0*   0*   0*   0*   0*   0* S  831  909 842  950  889  862  857  886  941 t  32  33  34  29  32  31  36  28  33P₁   0   0   0   0   0   0   0   0   0 P₂   0   0   0   0   0   0   0  0   0 F  510*  548*  516*  568*  538*  525*  523*  537*  564* r   0.82  0.77   0.82   0.75   0.81   0.82   0.80   0.77   0.77 R   0.79   0.78  0.80   0.77   0.78   0.78   0.80   0.77   0.77 Δ   0.229   0.204  0.228   0.177   0.210   0.195   0.216   0.181   0.123 D   0.197  0.157   0.211   0.143   0.174   0.180   0.205   0.150   0.130

TABLE 9 Ex. 40 41 42 43 44 45 46 47 48 SiO₂  73.0  72.6  73.4  71.7 74.3  71.0  72.3  72.6  72.0 Al₂O₃   6.0   7.0   7.0   8.1   6.9   7.0  6.9   7.5   7.5 MgO   6.0   7.0   5.0   6.1   5.9   8.0   7.9   7.0  7.0 CaO   0   0   0   0   0   0   0   0   0 ZrO₂   0   0   0   0   0  0   0   0   0 Na₂O  15.0  13.4  14.6  14.1  12.9  14.0  12.9  12.9 13.5 K₂O   0   0   0   0   0   0   0   0   0 Tg  588  631  604  629 633  625  636  643  636 T2 1660* 1684* 1692* 1692* 1712* 1657* 1681*1699* 1687* T4 1181* 1210* 1209* 1217* 1230* 1189* 1210* 1224* 1213* ρ  2.41*   2.406   2.40*   2.41*   2.39*   2.42*   2.41*   2.405   2.41 α 84*  78*  82*  81*  75*  81*  77*  76*  79* E  71*  69.9  71*  72*  72* 73*  73*  70.3  70 σ   0.2*   0.2   0.2*   0.2*   0.2*   0.2*   0.2*  0.2   0.2 P₀   0*   0*   0*   0*   0*   0*   0*   0*   0* S  750  943 791  884  855  860  912  924  898 t  31  29  35  28  26  24  20  22  24P₁   0   0   0   0   0   0   0   0   0 P₂   0   0   0   0   0   0   0  0   0 F  471*  565*  491*  536*  522*  524*  550*  555*  543* r   0.75  0.81 R   0.79   0.77   0.79   0.77   0.78   0.76   0.76   0.77   0.76Δ   0.271   0.162   0.241   0.190 D   0.224   0.140   0.215   0.136  0.170   0.109   0.103   0.116   0.120

TABLE 10 Ex. 49 50 51 52 53 54 55 56 57 SiO₂  71.6  73.6  72.0  72.2 72.0  72.0  72.5  73.0  72.5 Al₂O₃   7.0   8.0   7.0   6.8   6.5   6.8  6.5   6.0   6.2 MgO   8.0   5.0   7.5   7.5   8.0   8.0   8.0   8.5  8.5 CaO   0   0   0   0   0   0   0   0   0 ZrO₂   0   0   0   0   0  0   0   0   0 Na₂O  13.4  13.4  13.5  13.5  13.5  13.2  13.0  12.5 12.8 K₂O   0   0   0   0   0   0   0   0   0 Tg  630  634  633*  623 626  638*  637*  641*  627 T2 1669* 1723* 1721* 1717* 1701* 1720* 1724*1727* 1717 T4 1120* 1240* 1247* 1242* 1230* 1249* 1249* 1251* 1244 ρ  2.414   2.39*   2.41   2.41   2.41   2.41   2.41   2.40   2.41 α  79* 77*  79*  77  78  78*  77*  76*  74 E  71.5  71*  70  70  70  70  70 70  70 σ   0.2   0.2*   0.2   0.2   0.2   0.2   0.2   0.2   0.2 P₀   0*  0*   0*   0*   0*   0*   0*   0*   0* S  886  881  889  868  869  910 885  881  892 t  22  28  26  26  25  25  25  24  24 P₁   0   0   0   0  0   0   0   0   0 P₂   0   0   0   0   0   0   0   0   0 F  537*  534* 539*  528*  528*  549*  537*  535*  540* r   0.75 R   0.76   0.79  0.76   0.76   0.76   0.76   0.76   0.76   0.76 Δ   0.074 D   0.106  0.171   0.121   0.128   0.122   0.108   0.118   0.114   0.110

TABLE 11 Ex. 58 59 60 61 62 SiO₂  72.7  72.55  73.94  72.98  73.5 Al₂O₃ 6.3  8.2   7.65   8.25  7.6 MgO  8.5  4.6  4.4  4.6  4.4 CaO 0  0.6 0 0 0.6 ZrO₂ 0 0 0 0 0 Na₂O  12.5  14.1  13.98  14.19  13.9 K₂O 0 0 0 0 0Tg 639  629*  625*  628*  626*  T2 1732*  1706*  1718*  1716*  1708*  T41258*  1225*  1232*  1232*  1224*  ρ   2.40   2.41*   2.39*   2.40*  2.40* α 74  80* 79* 80* 79* E 71  71* 71* 71* 71* σ  0.2   0.2*   0.2*  0.2*   0.2* P₀  0*  0*  0*  0*  0* S 913  859  811  853  817  t 24 31* 33* 32* 32* P₁ 0 0 0 0 0 P₂ 0 0 0 0 0 F 550*  524*  500*  521* 503*  r R   0.75   0.77   0.80   0.79   0.78 Δ   0.258   0.269   0.272  0.341 D   0.103   0.257   0.206      0.282

TABLE 12 Ex. 1 2 3 4 5 6 7 8 9 SiO₂ 70.6 74.1 72.1 72.6 70.2 70.4 69.971.1 70.6 Al₂O₃ 11.5 8.2 8.4 8.4 11.4 11.5 11.4 10.1 11.7 MgO 3.9 3.95.3 6.7 3.5 3.6 3.5 6.6 6.6 CaO 0 0 0 0 0 0 0 0 0 ZrO₂ 0 0 0 0 1.0 0.61.3 0 0 Na₂O 14.0 13.9 14.3 12.3 13.9 13.9 13.8 12.2 11.1 K₂O 0 0 0 0 00 0 0 0

TABLE 13 Ex. 10 11 12 13 14 15 16 17 18 SiO₂ 70.4 71.6 72.1 69.8 68.667.0 68.5 69.7 76.5 Al₂O₃ 11.6 10.1 10.2 11.7 13.3 14.6 9.9 14.6 8.0 MgO5.9 8.0 9.4 5.3 3.9 4.5 6.7 3.8 3.8 CaO 0 0 0 0 0 0 0 0 0 ZrO₂ 0 0 0 0 00 1.3 0 0 Na₂O 12.1 10.3 8.3 13.2 14.2 13.8 12.1 11.8 11.7 K₂O 0 0 0 0 00 1.5 0 0

TABLE 14 Ex. 19 20 21 22 SiO₂ 59.5 69.5 82.2 79.5 Al₂O₃ 23.3 8.4 5.0 6.8MgO 3.1 10.0 0.7 6 CaO 0 0 0 0 ZrO₂ 0 0 0 0 Na₂O 14.1 7.16 12.1 6.1 K₂O0 2.95 0 1.6

TABLE 15 Ex. 23 24 25 26 27 28 29 30 SiO₂ 67.4 65.9 63.4 63.5 72.8 61.960.9 78.3 Al₂O₃ 14.9 9.8 9.7 11.4 1.9 17.1 9.6 5.2 MgO 2.6 6.8 6.9 7.13.7 3.9 7.0 8.2 CaO 0 0 0 0 8.1 0.6 0.0 0 ZrO₂ 0 2.5 3.7 0 0 0 4.8 0Na₂O 15.1 12.0 11.8 11.9 13.1 12.7 11.7 8.4 K₂O 0 3.0 4.5 6.0 0.3 3.55.9 0

TABLE 16 Ex. 31 32 33 34 35 36 37 38 39 SiO₂ 71.5 69.8 71.4 69.9 71.370.0 70.4 70.9 70.2 Al₂O₃ 10.7 12.3 11.5 11.5 11.4 11.5 13.1 9.9 13.1MgO 3.9 3.9 3.2 4.6 3.9 3.9 2.6 5.2 3.9 CaO 0 0 0 0 0 0 0 0 0 ZrO₂ 0 0 00 0 0 0 0 0 Na₂O 14.0 14.0 13.9 14.0 13.4 14.5 13.9 14.0 12.9 K₂O 0 0 00 0 0 0 0 0

TABLE 17 Ex. 40 41 42 43 44 45 46 47 48 SiO₂ 71.1 70.5 70.7 68.9 71.969.1 70.4 70.3 69.7 Al₂O₃ 9.9 11.5 11.5 13.2 11.4 11.6 11.5 12.3 12.3MgO 3.9 4.5 3.3 3.9 3.9 5.2 5.2 4.5 4.5 CaO 0 0 0 0 0 0 0 0 0 ZrO₂ 0 0 00 0 0 0 0 0 Na₂O 15.1 13.5 14.5 14.0 12.9 14.1 12.9 12.9 13.5 K₂O 0 0 00 0 0 0 0 0

TABLE 18 Ex. 49 50 51 52 53 54 55 56 57 SiO₂ 69.8 70.6 70.0 70.3 70.470.2 70.9 71.7 71.1 Al₂O₃ 11.5 12.9 11.6 11.2 10.8 11.3 10.8 10.0 10.3MgO 5.2 3.2 4.9 4.9 5.2 5.2 5.2 5.6 5.6 CaO 0 0 0 0 0 0 0 0 0 ZrO₂ 0 0 00 0 0 0 0 0 Na₂O 13.5 13.3 13.5 13.6 13.6 13.3 13.1 12.7 13.0 K₂O 0 0 00 0 0 0 0 0

TABLE 19 Ex. 58 59 60 61 62 SiO₂ 71.3 69.3 70.9 69.7 70.5 Al₂O₃ 10.513.3 12.4 13.4 12.4 MgO 5.6 2.9 2.8 2.9 2.8 CaO 0 0.5 0 0 0.5 ZrO₂ 0 0 00 0 Na₂O 12.6 13.9 13.8 14.0 13.8 K₂O 0 0 0 0 0

Further, 20 sheets of each of the glass plates having a size of 100mm×50 mm and a thickness of 1 mm and having a mirror-polished surface inExamples 1, 8, 28 and 29 were subjected to a chemical temperingtreatment. S was 700 MPa and t was 45 μm of the chemically temperedglass plate in Example 1, S was 700 MPa and t was 45 μm in Example 8, Swas 800 MPa and t was 40 μm in Example 28, and S was 650 MPa and t was55 μm in Example 29. To each of these chemically tempered glass plates,a force of x (unit: kgf) in Table 20 was applied by a Vickers indenter,and the breakage ratio (unit: %) was measured. Measurement was carriedout by a Knoop hardness meter FV-700 manufactured by FUTURE-TECH CORP.for an application time of 15 seconds and a pressing rate of 17 mm/sec.

From these measurement results, it is found that the glasses in Examplesof the present invention are hardly broken even when a Vickers indenterwas pressed under a high load.

TABLE 20 x Ex. 1 Ex. 8 Ex. 28 Ex. 29 5 0 0 30 0 10 0 0 100 0 20 20 80 —100 30 70 100 — 100

Further, 20 sheets of each of the glass plates having a size of 100mm×50 mm and a thickness of 1 mm and having a mirror-polished surface inExamples 1, 8, 28 and 29 were subjected to a chemical temperingtreatment. S was 700 MPa and t was 45 μm of the chemically temperedglass plate in Example 1, S was 700 MPa and t was 45 μm in Example 8, Swas 800 MPa and t was 40 μm in Example 28, and S was 650 MPa and t was55 μm in Example 29. To each of these chemically tempered glass plates,a force of x (unit: kgf) in Table 21 was applied by a Knoop indenter,and the breakage ratio (unit: %) was measured. Measurement was carriedout by a Knoop hardness meter FV-700 manufactured by FUTURE-TECH CORP.for an application time of 15 seconds and a pressing rate of 17 mm/sec.

From these measurement results, it is found that the glasses in Examplesof the present invention are hardly broken even when a Knoop indenterwas pressed under a high load.

TABLE 21 x Ex. 1 Ex. 8 Ex. 28 Ex. 29 5 0 10 10 10 10 0 60 100 60 20 0 90— 90 30 37 80 — 80 50 60 — — —

Further, with respect to Examples 1, 8 and 28, of the glasses which werenot broken by the above test, the indentation was measured. The length I(unit: μm) and the depth d (unit: μm) of the indentation are shown inTable 22.

From these measurement results, it is found that the glasses in Examplesof the present invention are hardly broken even when a great indentationdent is formed.

TABLE 22 x Ex. 1 l Ex. 1 d Ex. 8 l Ex. 8 d Ex. 28 l Ex. 28 d  5 387 13386 13 389 13 10 567 19 550 18 — — 20 805 27 — — — — 30 995 33 — — — —50 1135  38 — — — —

INDUSTRIAL APPLICABILITY

The glass for chemical tempering and the chemically tempered glass ofthe present invention are useful for e.g. a cover glass for a displaydevice. Further, they are useful also for e.g. a solar cell substrate ora window glass for aircrafts.

1. A glass, comprising, as represented by mole percentage based on thefollowing oxides: from 68 to 85% of SiO₂; from 3 to 15% of Al₂O₃; from 5to 15% of Na₂O; from 0 to 0.8% of K₂O; from 5 to 15% of MgO; and from 0to 1% of ZrO₂, wherein a total content of SiO₂ and Al₂O₃, SiO₂+Al₂O₃, isat most 88%.
 2. The glass of claim 1, comprising: from 68 to 80% ofSiO₂; and from 4 to 10% of Al₂O₃, wherein a total content SiO₂+Al₂O₃ isat most 85%.
 3. The glass of claim 1, which contains no CaO, or has aCaO content of less than 1% if contained.
 4. The glass of claim 1,comprising: at most 77% of SiO₂; and at least 8% of Na₂O; wherein atotal content SiO₂+Al₂O₃ is at most 85%, a content of CaO of less than1% if contained, and wherein the glass contains no K₂O.
 5. The glass ofclaim 1, wherein a total content SiO₂+Al₂O₃ is at least 75%.
 6. Theglass of claim 1, comprising at least 4.5% of Al₂O₃.
 7. The glass ofclaim 1, comprising: from 70 to 75% of SiO₂; at least 5% of Al₂O₃; atleast 8% of Na₂O; and from 5 to 12% of MgO, wherein a total contentSiO₂+Al₂O₃ is from 77 to 83%.
 8. The glass of claim 1, comprising atmost 6% of Al₂O₃.
 9. The glass of claim 1, comprising at least 8% ofMgO.
 10. The glass of claim 1, wherein a total content of CaO, SrO, BaO,and ZrO₂ is less than 1.5% if at least one of these four components iscontained.
 11. The glass of claim 1, wherein a total content of SiO₂,Al₂O₃, Na₂O, and MgO is at least 98%.
 12. The glass of claim 1, whereinwhen it is formed into a glass plate having a thickness of 1 mm andchemically tempered, and when a force of 98 N is applied to itsmirror-polished surface by means of a Vickers indenter, the probabilitythat the chemically tempered glass plate is broken is at most 10%. 13.The glass of claim 1, wherein when it is formed into a glass platehaving a thickness of 1 mm and chemically tempered, and when a force of196 N is applied to its mirror-polished surface by means of a Knoopindenter, the probability that the chemically tempered glass plate isbroken is at most 10%.
 14. The glass of claim 1, wherein Δ representedby the following formula is at most 0.21:Δ=(S₄₀₀-S₄₅₀)/S₄₀₀ wherein S₄₀₀ is a surface compressive stress obtainedwhen the glass is formed into a glass plate having a thickness of 1 mmand immersed in KNO₃ at 400° C. for 6 hours, and S₄₅₀ is a surfacecompressive stress obtained when the glass is formed into a glass platehaving a thickness of 1 mm and immersed in KNO₃ at 450° C. for 6 hours.15. A chemically tempered glass obtained by chemically tempering theglass of claim
 1. 16. The chemically tempered glass of claim 15, havinga compressive stress layer thickness of at least 10 μm, and a surfacecompressive stress of at least 400 MPa.
 17. A glass plate obtained bychemically tempering a glass plate comprising the glass of claim
 1. 18.A display device, comprising a cover glass comprising the glass plate ofclaim
 17. 19. The display device of claim 18, which is a mobile device,a touch panel, or a flat screen television having a size of at least 20inches.